Answer:
D. Single-stranded
Explanation:
DNA normally has a double-stranded structure. Each strand is composed by nitrogenous bases, sugars and phospates; strongly binded together by chemical bonds. The strands are bonded to each other, however, by relatively weak interactions between the hydrogen atoms in the nitrogenous bases in each strand. The energy supplied by heat is enough to break the weak interactions between hydrogens, but not enough to break the strong chemical bonds that make up the strands.
